This invention is generally related to image sensor integrated circuits and particularly those built using metal oxide semiconductor (MOS) fabrication processes.
Image sensor integrated circuits (ICs) are used in a wide variety of solid state electronic imaging systems. Many of these systems, including for instance still-shot cameras, also known as digital cameras, and certain video cameras, are equipped with MOS image sensor ICs rather than the more conventional charge coupled device (CCD) variety. An image sensor, whether it be a MOS or CCD variety, typically contains thousands of sensor elements sometimes known as picture element (pixel) circuits arranged in a flat, two-dimensional array. These sensor elements together form a photosensitive region that lies at the focal point of the camera and electrically captures an image that has been formed on the region. Cameras that use MOS image sensors have proven to be relatively inexpensive to manufacture as compared to those that have the CCD variety, and MOS-based cameras are expected to gain market share over their CCD rivals.
A problem with conventional MOS image sensors is that the relatively large size of a MOS image sensor element precludes the capture of images having high resolution, which has hampered their entry into certain markets that demand detailed images from the camera. FIG. 6 shows the equivalent circuit schematic of a conventional MOS image sensor element 604. The element 604 is typically connected to an output line 630 (also known as a bitline) to which a number of other identical sensor elements are also connected to form a column of an array (not shown). A bias transistor 626 is typically coupled to the bitline 630 as a load to each of the sensor elements. The array will include several thousand of such columns.
The conventional element 604 has a photodiode 608 which can be reset by asserting a control signal on a reset control line 612. The reset control line is typically coupled to several sensor elements, e.g., such as a row of elements in the array, so as to simultaneously prepare all of these elements to detect the incident energy. A reset transistor 614 receives the control signal at its gate and provides current from a main supply rail 616, having a potential Vdd, to reverse bias the photodiode to a reset voltage. A photodiode signal is amplified by a source follower 618 whose current is supplied by a pixel supply rail 620 at a potential VccPx, which is typically separate from the supply rail 616. The main and pixel supply rails typically extend across the sensor array and connect to each sensor element in the array.
The incident energy, typically visible light, is detected for the entire array during a single time interval known as the exposure or integration interval in which a photodiode signal is generated. The integration interval begins when reset is deasserted, and the photodiode is presented a high impedance by the reset transistor. This allows the photodiode voltage to decay in proportion to the incident energy and instantaneously yield a photodiode signal voltage. While a select signal is asserted on the select control line 610, the signal voltage is sampled at the end of the integration interval. The difference between the signal voltage and the reset voltage is a measure of the detected incident energy during the integration interval.
In view of the foregoing, it would be desirable to increase the image resolution of such a sensor array, without increasing the size of the integrated circuit die on which the sensor array is formed, and without adversely impacting sensor performance.
In accordance with an embodiment of the invention, an image sensor is disclosed. The sensor has a number of sensor elements, where each element has a first switch coupled to a photodetector to alternatively, under the control of a reset signal, (1) provide a first current to reset the photodetector, and (2) present a high impedance to the photodetector. A reset line is coupled to the first switch in each of the sensor elements to provide the reset signal, and to both control the first switch and supply the first current.
Other features and advantages of the invention will be apparent from the accompanying drawings and from the detailed description that follows below.